Stimulated emission of phonons in an acoustic cavity

This thesis will present experiments on stimulated emission of phonons in dilute ruby following complete population inversion of the Zeeman-split E(2E) Kramers doublet by selective pulsed optical pumping into its upper component. The resulting phonon avalanches are detected by use of the R1 luminescence emanating from the inverted zone, located near the end face where the laser beam enters the crystal. The phonons appear to team up into a highly directional phonon beam. The phonon frequency is tunable from, say, 10-100 GHz via the magnetic field splitting of the doublet. Remarkably, the population of the lower doublet component, which is a measure of the number of phonons generated, evolves with a sequence of distinct steps. The time interval in between these steps equals 2L/v, corresponding to the time the phonons need to return to the inverted zone by reflection at the opposite end face at a distance L. The end faces of the ruby crystal thus form an acoustic cavity. The phonon beam passes the inverted zone repeatedly to be amplified further, in a manner similar to light in an optical laser. In other words, the basic ingredients for a phonon laser have been established